Mesh collapses too much when DynTopo sculpting
I took a look at your file. It seems your brush density was too drastically different from your mesh density, and this was causing some extreme pinching and decimation.

Try using Constant Detail instead (which I find is almost always best anyway because you can maneuver around the 3D Viewport without your sculpting being affected), and then use the Eyedropper to click somewhere on your mesh to sample its density. Then your brush resolution will match your mesh density.



Result:



There are some other settings you can play with that may give you even better results. Here is a sample .blend you can check out and experiment with.



Finally, here is another answer worth looking at to further your understanding of the Dyntopo settings.